Current switching sensor detector

ABSTRACT

A sensor for a switching circuit detects the logical state of the switching circuit by monitoring the current flow through the switching circuit. The current flow is conditioned by one or more current limiters and a voltage regulator, coupled in series with the switching circuit. The sensor also includes a current limit control circuit coupled to each of the current limiters. The sensor is effectively shielded from the effect of parasitic capacitance in the switching device because the current flow through the switching circuit reacts immediately and without regard to the level of parasitic capacitance whenever the switching circuit makes a state change.

FIELD OF INVENTION

[0001] The present invention relates to sensing a logic state using asensor or detector. More specifically, the present invention relates toa current sensing architecture for detecting a logic state.

BACKGROUND OF THE INVENTION

[0002] One way to detect the logic state of a switching device is tocouple the device between a power source and ground and measuring theresulting voltage. For example, in FIG. 1A, power is applied at terminal101, which is coupled in series with a resistor 102 and a switchingdevice 104 to a ground 105. The switching device 104 may be a singleswitching device, such as a transistor, or a more complex device, suchas a series of switching devices which form a logic circuit having alogic output. The logic state of the switching device 104 may bedetermined by measuring the voltage at terminal 103. If the voltage atterminal 103 is relatively high, then the switching device 104 is in aopen state. Similarly, if the voltage at terminal 103 is relatively low,then the switching device 104 is in a closed state. The change involtage at terminal 103 is related to the current flow rate through theswitching device. Thus, the voltage sensing at terminal 103 should beperformed only after the sufficient time has elapsed for the voltage tobecome stable after a state change in the switching device 104.

[0003] An issue which arises when using a circuit such as illustrated inFIG. 1A in a semiconductor device is that of parasitic capacitance.Parasitic capacitance is a unwanted capacitance resulting from thefabrication of the semiconductor device and is typically associated withconductive lines. FIG. 1B illustrates a circuit equivalent to thatillustrated in FIG. 1A, but with the parasitic capacitance illustratedexplicitly illustrated as capacitor 106 coupled in parallel to theswitching device 104 in-between resistor 102 and ground 105. The effectof parasitic capacitance is to reduce the rate a voltage at node 103changes over time as the switching device 104 switches states. Forexample, if the switching device 104 were open and then switched to aclose position, the voltage a node 103 in FIG. 1B would fall towards itsnew value at a slower rate than if the parasitic capacitance 106 werenot present. Parasitic capacitance, therefore, increases the timerequired to detect a changed state of the switching device 104.

[0004] One method for compensating the reduced switching speed imposedby parasitic capacitance is to provide increased current flow throughthe circuit. Increasing the maximum current flow through the switchingdevice 104 discharges the charge stored by the parasitic capacitancefaster when switch 104 is closed and changes capacitor 106 faster whenswitch 104 is opened. Thus, increasing the maximum current flowthroughout the circuit permits the voltage at node 103 to reach a stablestate faster after the switching device 104 has changed its logicalstate. Unfortunately, increasing the maximum current flow also increasesthe power consumption of the circuit. Accordingly, there is a need anddesire for a method and apparatus to quickly and efficiently detect alogic state of a device in an environment having significant parasiticcapacitance.

SUMMARY OF THE INVENTION

[0005] The present invention is directed to an apparatus and method forquickly and efficiently detecting a logic state of a switching device.The present invention incorporates a series circuit coupling a powersupply source to ground through a current sensing amplifier, at leastone current limiter, a voltage regulator, and the switching device. Acurrent limiter control circuit is coupled to the at least one currentlimiter. In an alternate embodiment, two current limiters are used inthe series circuit. The current sensing amplifier measures the currentflowing through the switching device and does not need to wait forcharge stored by the parasitic capacitance to charge or discharge beforesensing a logic level change. Thus, the present invention is not slowedby parasitic capacitance and does not require increased current flow tocompensate for the parasitic capacitance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The foregoing and other advantages and features of the inventionwill become more apparent from the detailed description of exemplaryembodiments of the invention given below with reference to theaccompanying drawings in which:

[0007]FIG. 1A is a circuit diagram of a conventional voltage detectingcircuit for a switching device;

[0008]FIG. 1B is a circuit diagram of a conventional voltage detectingcircuit switching device in an environment having parasitic capacitance;

[0009]FIG. 2 is a block diagram of one embodiment of the presentinvention;

[0010]FIG. 3 is a illustration of the current sensing circuit;

[0011]FIG. 4 is a illustration of a current limiter control circuit;

[0012]FIG. 5 is an illustration of an alternate embodiment of thecurrent sensing circuit; and

[0013]FIG. 6 is a block diagram of a CAM memory array having CAM cellswhich incorporate the match detection circuitry of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0014] Now referring to the drawings, where like reference numeralsdesignate like elements, there is shown in FIG. 2 a block diagram of thepresent invention 200 coupled to a switching device 104. The presentinvention includes several components coupled in series between a powersource (Vdd) and the switching device 104, and between the switchingdevice 104 and a ground potential. More specifically, a current sensingamplifier 201, a first current limiter 203, and a voltage regulator 205are coupled in series between the power source and the switching device104. Additionally, a second current limiter 204 is coupled between theswitching device 104 and the ground potential. In an alternateembodiment, the second current limiter 204 is not used. The presentinvention also includes a current limit control 202, which is coupled tothe first and second current limiters 203, 204. The switching device 104is shown as a switch, e.g., a transistor switch, however, the switchingdevice 104 may be other devices or circuits which act as a logic levelswitch.

[0015] The present invention 200 operates by detecting changes in thecurrent (Is) at current sensing amplifier 201 flowing into a firstcurrent limiter 203, the output of which is applied to the voltageregulator 205 which supplies a voltage regulated current to the switchdevice 104. The switch device 104 is also optionally connected to thesecond current limiter 204 to ground. As will be explained in greaterdetail below, the first and second current limiters 203, 205 cooperatewith the current limit control circuit 202 to maintain the voltage (Vs)at the upper node of the switching device 104 at a predetermined value.

[0016] In a steady state with the switching device 104 in an open state,no current flows through the switching device 104 or the voltageregulator 205. As the switching device 104 transitions to a closedstate, discharge current begins to flow through the switching device104. A portion of this discharge current is caused by the charge storedin the parasitic capacitance (FIG. 2, element 106). At the same time,the voltage regulator 205 attempts to maintain the voltage Vs bysupplying a charging current. If the discharge current through theswitching device 104 is not limited, the magnitude of the dischargecurrent would be equal to the voltage Vs maintained by the voltageregulator 205 divided by the impedance of the switching device 104 inits closed state.

[0017] The FIG. 2 embodiment of the invention includes two currentlimiters 203, 204 controlled by the current limit control 205 circuit tolimit current flow and thereby control power usage. In particular, thefirst current limiter 203 is used to limit the charging current suppliedby the voltage regulator 205, while the second current limiter 204 isused to limit the discharge current through the switching device 204. Inthe preferred embodiment, the two current limiters 203, 204 arecontrolled by the current limit control 202 circuit to allow equalamounts of charge and discharge currents to flow, thereby permitting thevoltage Vs to be set at a predetermined level. The predetermined levelis ideally a low level, in order to minimize the amount of charge storedby the parasitic capacitance.

[0018]FIGS. 3 and 4 are circuit diagrams illustrating the FIG. 2exemplary embodiment of the present invention. More specifically, FIG. 3shows an implementation of the current sensing amplifier 201, the firstand second current limiters 203, 204, and the voltage regulator 205,while FIG. 4 illustrates the current limit control circuit 202.

[0019] As shown in FIG. 3, the current sensing amplifier 201 can beconstructed as a circuit having five transistors. More specifically,transistors 301, 302 form a current mirror whereby current flows throughtransistors 302 mirrors the current flow through transistor 301, whiletransistors 303, 305 form an invertor/comparator which converts thevoltage on the drain of transistor 308 to the logic signal DATA. Thetransistor 308, which also has its source coupled to a ground potentialand its gate coupled to a DISCHARGE pin, is used to discharge any chargestored within the current sensing amplifier 201 due to its own parasiticcapacitance.

[0020] The first and second current limiters 203, 204 are implemented asa single transistor acting as a variable resistor. The first currentlimiter includes transistor 304 which has its gate voltage controlled bythe signal CLREF1, while the second current limiter 204 includestransistor 307, which has its gate voltage controlled by the signalCLREF2. The CLREF1 and CLREF2 signals are governed by the current limitcontrol 202 circuit, explained below with reference to FIG. 4.

[0021] The voltage regulator 205 is also implemented using a singletransistor 306. The transistor 306 has its drain coupled to the drain oftransistor 304. The transistor 306 has its gate voltage coupled to a DCvoltage reference signal VREF. The output impedance of the transistor306, at the SENSE pin, is low.

[0022] Now referring to FIG. 4, the current limit control circuit 202may be formed from opamps 401, 402, resistors 403, 404, and transistors309-314. A fixed, temperature stable reference voltage is supplied atthe VREF pin. This reference voltage is applied to both opamps 401, 402.When the circuit 202 is settled, the voltage levels on the inverting(Ain\) pin of each opamp 401, 402 must be equal to the non-inverting(Ain) pin. Under these conditions the voltage across resistor 403 equalsthe voltage across resistor 404. In the preferred embodiment theresistance of both resistors 403, 404 are equal, causing the currentwhich flows through both resistors 403, 404 to be equal as well. Thecurrent which flows through resistor 403 is generated by a currentmirror formed by transistors 309, 311. The current through transistor311 and 309 are equal and controlled by transistor 313 and opamp 401.The current through transistor 313 equals the current through resistor403. The voltage which controls transistor 313 is coupled to CLREF1.

[0023] Similarly, the current which flows through resistor 404 isgenerated by a current mirror formed by transistors 312, 310. Thecurrent through transistor 311 and 309 are equal and controlled bytransistor 314 and opamp 402. The current through transistor 314 equalsthe current through resistor 404. The voltage which controls transistor314 is coupled to CLREF2. In the preferred embodiment the CLREF1 andCLREF2 signals are set so that the current limit in the first and secondcurrent limiters 203, 204, i.e., the current through transistors 304,307 are equal, and there is net no current which would charge ordischarge the parasitic capacitance. A possible modification to thecurrent limiter control circuit 202, for use in connection with analternate embodiment utilizing a single current limiter, is describedbelow in connection with FIG. 5.

[0024] Referring again to FIG. 3, the switching device 104 is coupledbetween the SENSE and LIMIT pins. The parasitic capacitance can bethought of as a capacitor coupled between the SENSE pin and ground. Whenthe switching device is in a closed state, current will flow from theSENSE pin through the switching device 104 to the LIMIT pin.

[0025] The switching device 104 is coupled between the SENSE pin and theLIMIT pin (in the embodiment using both current limiters 203, 204) orbetween the SENSE pin and ground (in the embodiment using only currentlimiter 203). Under either embodiment, the parasitic capacitance of theswitching device 104 can be thought of as a capacitor coupled betweenthe SENSE pin and the ground. When the switching device 104 is in aclosed state, current will flow from the SENSE pin through the switchingdevice 104 to the limit pin. If the current limiters 203, 204 are set tothe same current limit, the parasitic capacitance of the switchingdevice 104 will not be charging or discharging. Thus, the voltage at thesense pin will remain constant.

[0026] At the current sensing amplifier 201, the DISCHARGE pin isnormally kept at a low logic level. The transistor 308 is thereforebehaves like an open circuit, and permits the small current generated bytransistor 302 to rapidly charge the parasitic capacitance associatedwith the current sensing amplifier 201 (i.e, transistors 301, 302, 303,305, 308).

[0027] When the switching device 104 moves from a closed state to anopen state, no current can flow through the voltage regulator 305 (i.e.,transistor 306). Additionally, since the parasitic capacitanceassociated with both the switching device 104 and the current sensingamplifier 201 are charged, no current flows due to the parasiticcapacitance. Thus, the output produced by the current sensing amplifier201 at the DATA pin is stable and corresponds to the switching device104 being in a open state.

[0028] After one (and before the next) current sensing operation, theparasitic capacitance of the current sensing amplifier 201 must bedischarged. This may be done by temporarily placing a high level signalon the DISCHARGE pin, which causes transistor 308 to behave like anclosed circuit, permitting the charge stored in the parasiticcapacitance to flow to ground through transistor 308. Since theparasitic capacitance of the current sensing amplifier 201 is lowrelative to the parasitic capacitance of the switching device 104, theparasitic capacitance of the current sensing amplifier 201 may becharged or discharged quickly. The state of the DISCHARGE pin isnormally toggled high for a brief period of time as the switching device104 changes state. The output at the DATA pin of the current sensingamplifier 201 is stable a short time after the state of the DISCHARGEpin returns low after being toggled high as the switching device 104changes states.

[0029] When the switching device 104 moves to a closed state from anopen state, a current begins to immediately flow through the switchingdevice 104. A portion of this current flow is from the voltage regulator205, as the voltage regulator attempts to maintain the voltage at theSENSE pin at a predetermined value. Another portion of the current flowis a discharge current from the parasitic capacitance. The portion ofthe current which flows through the voltage regulator 205 also flowsthrough the first current limiter 203 and the current sensing amplifier201. The current flow through transistor 301 is mirrored in transistor302 and is quickly output as a signal on the DATA pin by inverter 303,305.

[0030]FIG. 5 illustrates an alternate embodiment which does not utilizethe second current limiter 204. This alternate embodiment features thesame circuitry for the current sensing amplifier 201, the first currentlimiter 203, and the voltage regulator 205. However, since the secondcurrent limiter 204 has been removed, the switching device 104 iscoupled between the SENSE pin and a source of ground potential. In thisembodiment, the CLREF2 signal is not used since there is only a singlecurrent limiter 203, which is controlled by the CLREF1 signal. Althoughthe current limiter control circuit 202 illustrated in FIG. 4 may alsobe used in this embodiment, since it generates control signals CLREF1,CLREF2, in the interest of efficiency the circuit of FIG. 4 may bemodified by eliminating opamp 402, resistor 404, transistors 310, 312,and 314, and node CLREF2. The resulting circuit would then only generatethe CLREF1 control signal, which is all that is needed in the singlecurrent limiter embodiment.

[0031] The present invention may be used in any application whereparasitic capacitance may be a concern. For example, one suchapplication may be in content addressable memory systems. Referring nowto FIG. 6, a portion of a CAM memory array 600 using the presentinvention is illustrated. The CAM array 600 includes a plurality of CAMcells 601 which are arranged in rows 602 and columns 603. Each CAM cell601 includes a match detection circuit 200 which may employ the logicstate detector of the present invention. CAM cells 601 are coupled toeach column 602 via complementary DATA 610 and DATA* 611 lines.Similarly, the CAM cells 601 are coupled to each row via a word line 620and a match line 630.

[0032] In a CAM, each stored data word may be searched against a targetdata pattern. For example, the search data may be placed upon the DATA610 and DATA* 611 lines. A search is conducted simultaneously on alldata words in the CAM. The match detection circuit 200 of the presentinvention may be used to detect the match between data on the datalines, and stored data. If the stored and search data do not match, thematch line (which is pre-charged before the search data is asserted onthe DATA 610 and DATA* 611 lines) is discharged through the cell 601.Thus, the match line 620 remains high only when the entire word matchesthe search data.

[0033] While the invention has been described in detail in connectionwith the exemplary embodiment, it should be understood that theinvention is not limited to the above disclosed embodiment. Rather, theinvention can be modified to incorporate any number of variations,alternations, substitutions, or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Accordingly, the invention is not limited by the foregoingdescription or drawings, but is only limited by the scope of theappended claims.

1-28 (cancelled)
 29. A circuit for detecting a state of a controllableelement, comprising: a current sensing amplifier for producing a firstcurrent; a first current limiter, coupled to said current sensingamplifier, for limiting a current level of a current flowing throughsaid first current limiter to be no greater than a first parameter; anda voltage regulator, coupled to said first current limiter, forcontrolling a voltage at an output node of said voltage regulator to beat a second parameter; wherein said first current flows in seriesthrough said first current limiter, said voltage regulator, and saidcontrollable element, said current sensing amplifier senses a currentlevel of said first current and produces an output signal as a functionof said current level, said output signal also being indicative of astate of said controllable element.
 30. The circuit of claim 29, whereinsaid controllable element is a switching circuit.
 31. The circuit ofclaim 29, further comprising: a second current limiter, coupled to saidcontrollable element, for limiting a current level of a current flowingthrough said second current limiter to be no greater than a thirdparameter; wherein said first current also flows in series through saidsecond current limiter.
 32. The circuit of claim 31, wherein said firstcurrent flows from said first current limiter to said controllableelement, and from said controllable element to said second currentlimiter.
 33. The circuit of claim 29, further comprising: a controlcircuit, coupled to said current sensing amplifier, for controlling alevel of said first parameter.
 34. The circuit of claim 33, wherein saidcontrol circuit is also coupled to said voltage regulator and controls alevel of said second parameter.
 35. The circuit of claim 33, furthercomprising: a second current limiter, coupled to said controllableelement, for limiting a current level of a current flowing through saidsecond current limiter to be no greater than a third parameter; whereinsaid first current also flows in series through said second currentlimiter and said control circuit also controls a level of said thirdparameter.
 36. The circuit of claim 35, wherein said control circuit isalso coupled to said voltage regulator and controls a level of saidsecond parameter.
 37. The circuit of claim 36, wherein said controlcircuit maintains a same level for said first and third parameters. 38.The circuit of claim 29, wherein said current sensing amplifiercomprises: an input node for receiving an input power; a current mirrorhaving an input coupled to said input node, a first output forgenerating said first current, and a second output for generating asecond current; and a output circuit, said output circuit producing saidoutput signal from said second current.